Add triode/pentode switch to Valve Jr

[dunkelfalke]

Hello forum

I have bought a used Harley Benton GA5 tube amp a couple of weeks ago (pretty much the same as the Valve Jr, except the added tone knob).

Two days ago it started to shock me with some low voltage through the guitar strings. I opened the amp and saw that one fuse holder was broken open and the fuse fell off.

So I thought, since the amp has to be repaired anyway, why not to mod it a little bit because it only starts to saturate at the maximum volume, but the maximum volume is too loud for a city flat.

I have heard about a triode/pentode switch, that I can switch the 12AX7 preamp tube in the triode mode and then the amp would saturate at lower volumes.

Is it true and if yes, can someone show me how to do it?

Thanks in advance

[ChrisK]

dunkelfalke,

Where have you been? Willkommen zuruck (yeah, I know...).

Hmmm, triode mode is the only mode available in a 12AX7 since it is, uh, a triode. Did you mean the power tube which I assume in a pentode?

Triode mode has a sooner, smoother occurrence of distortion/clipping that a pentode which has a quicker onset, later. It also usually occurs at, and results, in less output power.

The power output tube type is an EL84

I have some schematics and/or articles for a triode/pentode switch and will look for them as they are on the laptop that I have with me. Give me a few minutes.

Ok, I found them but they are no longer available online. Check your mailbox, I PM'd them to you.


Hey, update your email address on guitarnuts2.

[dunkelfalke]

oh, yes, thanks for the correction

the email address in the profile is up to date now.

i was absent for quite a long time because i lost my job and nearly lost my girlfriend, so i had no time to login to guitarnuts and even no time to play guitar at all so i almost forgot how to play and am trying to start again now.

by the way, except for the omitted umlaut dots (lacking the german keyboard) your german was perfect.

[ChrisK]

by the way, except for the omitted umlaut dots (lacking the German keyboard) your German was perfect.

Well, it can be done, I'm just too lazy to do so.

I will resend the documents for the triode/pentode switch.

Oh, and

[newey]

zurück ist . . .

ChrisK- Too lazy for a simple umlaut but not too lazy for the Cyrillic alphabet I see!

Dunklefalke- Good to see you back!

From what I understand, the Valve Jr. chassis is identical to yours, the only difference is the tone pot- which is a popular mod for VJ owners.

Replacing the output transformer seems to also be a popular mod for the Valve Jr.

[drstuart]

Hi ChrisK!
Any chance of a copy of the schematics for a triode/pentode switch? I'm looking to mod a little Crate V8 palomino, I think I've got it right but I'm a bit concerned about the screen grid resistor I'll need to add. The V8 has an EL84 as output valve, you probably know this!
Btw, have you had any experience with Power Scaling as offered by London Power in Canada?:
www.londonpower.com/pscaling.htm

Regards
Stuart

[newey]

Dr Stuart:

Hello and Welcome!

I'm sure Chris will be along soon to help with your project.

We have had discussions before about power scaling, attenuators and so forth, including in this thread:

Reducing Tube Amp Volume without affecting tone

[ChrisK]

Triode/Pentode switch design note is attached.

www.ax84.com/

www.ax84.com/buildingblocks.html

Btw, have you had any experience with Power Scaling as offered by London Power in Canada?

No.


P.S. I tried attaching the file. I can't open it when I read my post. If you can't, PM me and I'll send it (drstuart only).

Attachments: